Epitaxial film forming method, sputtering apparatus, manufacturing method of semiconductor light-emitting element, semiconductor light-emitting element, and illumination device

ABSTRACT

The present invention provides an epitaxial film forming method for epitaxially growing a high-quality group III nitride semiconductor thin film on an α-Al 2 O 3  substrate by a sputtering method. In the epitaxial film forming method according to an embodiment of the present invention, when an epitaxial film of a group III nitride semiconductor thin film is to be formed on the α-Al 2 O 3  substrate arranged on a substrate holder provided with a heater electrode and a bias electrode of a sputtering apparatus, in a state where the α-Al 2 O 3  substrate is maintained at a predetermined temperature by the heater electrode, high-frequency power is applied to a target electrode and high-frequency bias power is applied to a bias electrode and at that time, the powers are applied so that frequency interference between the high-frequency power and the high-frequency bias power does not occur.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/919,036, filed on Jun. 17, 2013, which is a continuation applicationof PCT International Application No. PCT/JP2011/007040, filed on Dec.16, 2011, which claims the benefit of priority from Japanese ApplicationNo. 2010-289265, filed Dec. 27, 2010. The contents of the aforementionedapplications are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to an epitaxial film forming method, asputtering apparatus, a manufacturing method of a semiconductorlight-emitting element, a semiconductor light-emitting element, and anillumination device and particularly to an epitaxial film forming methodcapable of forming a high-quality epitaxial film, a manufacturing methodof a semiconductor light-emitting element using such epitaxial film, asputtering apparatus, a semiconductor light-emitting element, and anillumination device.

BACKGROUND ART

Group III nitride semiconductors are compound semiconductor materials ofan aluminum (Al) atom, a gallium (Ga) atom, and an indium (In) atomwhich are group IIIB elements (hereinafter referred to simply as IIIelements) and a nitrogen (N) atom which is a group VB element(hereinafter referred to simply as a group V element). That is, compoundsemiconductor materials obtained as aluminum nitride (AlN), galliumnitride (GaN), indium nitride (InN) and their mixed crystals (AlGaN,InGaN, InAlN, InGaAlN) are group III nitride semiconductors.

Elements using the group III nitride semiconductors include opticaldevices such as light-emitting diodes covering a wide wavelength regionover far-ultraviolet/visible/near-infrared regions (LED: Light EmittingDiode), laser diode (LD), solar cell (PVSC: Photovoltaic Solar Cell),photodiode (PD) and the like and electronic devices such as highelectron mobility transistor for high-frequency/high-output applications(HEMT: High Electron Mobility Transistor), metal-oxide-semiconductorfield effect transistor (MOSFET) and the like.

In order to realize element applications as above, it is necessary toepitaxially grow a group III nitride semiconductor thin film on a singlecrystal substrate so as to obtain a high-quality single crystal filmwith few crystal defects (epitaxial film). However, since a singlecrystal substrate made of a group III nitride semiconductor is extremelyexpensive, it is not used except some applications, and a single crystalfilm is obtained mainly by hetero-epitaxial growth of sapphire(α-Al₂O₃), silicon carbide (SiC) on dissimilar substrates. Particularly,α-Al₂O₃ substrates are inexpensive and those with a large area and highquality can be obtained, and therefore almost all of LEDs using thegroup III nitride semiconductor thin film sold in the market use α-Al₂O₃substrates.

For the epitaxial growth of such group III nitride semiconductor thinfilm, a metal organic chemical vapor deposition (MOCVD) method by whichhigh-productivity and high-quality epitaxial films can be obtained isused. However, the MOCVD method has problems that a production cost ishigh, particles can be easily generated and it is difficult to obtainhigh yield and the like.

On the other hand, a sputtering method has features that, a productioncost can be suppressed low and particle generation probability is alsolow. Therefore, if at least a part of a film forming process of thegroup XII nitride semiconductor thin film can be replaced by thesputtering method, it is likely that at least a part of theabove-described problems can be solved.

However, the group III nitride semiconductor thin film fabricated byusing the sputtering method has a problem that a crystal quality tendsto become poorer than those fabricated by using the MOCVD method. Forexample, crystalline of the group III nitride semiconductor thin filmfabricated by using the sputtering method is disclosed in NPL 1, forexample. In NPL 1, a GaN film with c-axis orientation is epitaxiallygrown on an α-Al₂O₃ (0001) substrate by using a high-frequency magnetronsputtering method. NPL 1 describes that in X-ray rocking curve (XRC)measurement of a GaN (0002) plane, its full width at half maximum (FWHM)is 35.1 arcmin (2106 arcsec), This value is an extremely large value ascompared with a GaN film on the α-Al₂O₃ substrate currently sold in themarket and indicates that mosaic expansion of tilt which will bedescribed later is large and crystal quality is poor.

Here, concepts used as indexes indicating the crystal quality, that is,(1) mosaic expansion of tilt, (2) mosaic expansion of twist, and (3)polarity will be described in brief. The mosaic expansion or tilt in (1)indicates a degree of variation in crystal orientation in a substrateperpendicular direction, and the mosaic expansion of twist in (2)indicates a degree of variation in crystal orientation in a substratein-plane direction. The polarity in (3) is a term meaning an orientationof a crystal, and in the case of c-axis orientation film, there are twotypes of growth modes, that is, +c polarity and −c polarity. The growthwith the +c polarity corresponds to (0001) orientation and the growthwith −c polarity corresponds to (000-1) orientation.

It is necessary for a single crystal with favorable crystalline thatmosaic expansions of tilt and twist are small and also, polarity isbiased to either of the +c polarity or the −c polarity. Particularly,since a group III nitride semiconductor thin film with favorablemorphology and excellent crystalline can be easily obtained with the +cpolarity, establishment of a process of obtaining group III nitridesemiconductors with the +c polarity is in demand. On the other hand,many attempts have been made in order to obtain a good-quality group IIInitride semiconductor thin film by the sputtering method (See PTLs 1 and2).

PTL 1 discloses a method of realizing higher quality of group IIInitride semiconductor thin films by applying plasma processing to asubstrate before a group III nitride semiconductor thin film (AlN inPTL 1) is formed on an α-Al₂O₃ substrate by using the sputtering methodor particularly a method of obtaining a group III nitride semiconductorthin film with extremely small mosaic expansion of tilt.

Moreover, PTL 2 discloses a manufacturing method of a group III nitridesemiconductor (group III nitride compound semiconductor in PTL 2)light-emitting element in which a buffer layer (an intermediate layer inPTL 2) made of the group III nitride semiconductor (group III nitridecompound in PTL 2) is formed on a substrate by the sputtering method andan n-type semiconductor layer provided with a base film, alight-emitting layer, and a p-type semiconductor layer are sequentiallylaminated on the buffer layer made of this group III nitridesemiconductor.

PTL 2 describes that, as a procedure for forming the buffer layer madeof the group III nitride semiconductor, a pre-treatment process ofapplying plasma processing to a substrate and a process of depositingthe buffer layer made of the group III nitride semiconductor bysputtering method subsequently to the pre-treatment process. Moreover,in PTL 2, as a preferable mode of the substrate and the buffer layermade of the group III nitride semiconductor, an α-Al₂O₃ substrate andAlN are used, and as a deposition method of the n-type semiconductorlayer provided with a base film, the light-emitting layer, and thep-type semiconductor layer, the MOCVD method is preferably used.

CITATION LIST Patent Literature

-   PTL 1: International Publication No. 2009/096270-   PTL 21 Japanese Patent Application Laid-Open No. 2008-109084

Non Patent Literature

-   NPL 1: Y. Daigo, N. Mutsukura, “Synthesis of epitaxial GaN    single-crystalline film by ultra high vacuum r.f. magnetron    sputtering method”, Thin Solid Films 483 (2005) p 38-43.

SUMMARY OF INVENTION

According to the already disclosed prior-art technologies (PTL 1 and PTL2), a group III nitride semiconductor having small mosaic expansion oftilt or twist is obtained by the sputtering method. However, theprior-art technologies do not disclose a method of controlling polarity,and there is a serious problem in employing the sputtering method as amanufacturing process of a group III nitride semiconductor.

Actually, when an AlN film was formed on an α-Al₂O₃ substrate by thesputtering method by using the technologies disclosed in PTLs 1 and 2,an AlN film with small mosaic expansion of tilt or twist could beobtained but the +c polarity and the −c polarity were mixed with regardto the polarity. Moreover, when a GaN film was grown on the AlN film inwhich the +c polarity and the −c polarity were mixed by the MOCVDmethod, a high-quality GaN film could not be obtained. Moreover, alight-emitting element was fabricated by using the obtained GaN film,but favorable emission characteristics could not be obtained. Therefore,mixture of the +c polarity and the −c polarity is not reduced and agroup III nitride semiconductor thin film with the +c polarity cannot beobtained only by the technologies disclosed in PTLs 1 and 2. That is,though the technologies disclosed in PTLs 1 and 2 are effectivetechnologies since the mosaic expansion of tilt or twist can be madesmall, unification of polarity as much as possible is in demand in orderto obtain a group III nitride semiconductor thin film with a higherquality.

The present invention is made in view of the above-described problemsand has an object to provide an epitaxial film forming method capable offabricating an epitaxial film with improved degree of unification of the+c polarity (improved (0001) orientation) and further to provide amanufacturing method of a semiconductor light-emitting element usingthis epitaxial film, a sputtering apparatus, a semiconductorlight-emitting element manufactured by this manufacturing method, and anillumination device.

The inventors have obtained new founding as the result of keenexamination that polarity of an epitaxial film can be controlled byhigh-frequency bias electric power applied to a bias electrode built ina substrate holder as will be described later and completed the presentinvention.

In order to achieve the above-described object, a first aspect of thepresent invention is an epitaxial film forming method that uses asputtering apparatus having a target electrode on which a target can bearranged and a substrate holder on which a substrate can be arrangedtoward the target electrode and provided with a heater electrode and abias electrode, and that epitaxially grows a group III nitridesemiconductor thin film by a sputtering method on an α-Al₂O₃ substratearranged on the substrate holder, the method comprising the steps of;arranging the α-Al₂O₃ substrate on the substrate holder; and forming anepitaxial film of the group III nitride semiconductor thin film on theα-Al₂O₃ substrate arranged on the substrate holder, wherein the step offorming an epitaxial film of the group III nitride semiconductor thinfilm maintains the α-Al₂O₃ substrate at a predetermined temperature bythe heater electrode, and applies high-frequency power to the targetelectrode and also applies high-frequency bias power to the biaselectrode, and wherein the high-frequency power and the high-frequencybias power are applied so that frequency interference between thehigh-frequency power and the high-frequency bias power does not occur.

Moreover, a manufacturing method of a semiconductor light-emittingelement according to a second aspect of the present invention has a stepof forming a buffer layer of the semiconductor light-emitting element bythe method of forming an epitaxial film according to the above-describedfirst aspect.

Moreover, a third aspect of the present, invention is a semiconductorlight-emitting element in which at least a buffer layer, a group IIInitride semiconductor intermediate layer, an n-type group III nitridesemiconductor layer, a group III nitride semiconductor active layer, ap-type group III nitride semiconductor layer, and a translucentelectrode are formed on the α-Al₂O₃ substrate, wherein at least onelayer of the buffer layer, the group III nitride semiconductorintermediate layer, the n-type group III nitride semiconductor layer,the group III nitride semiconductor active layer, and the p-type groupIII nitride semiconductor layer is fabricated by the epitaxial filmforming method according to the above-described first aspect. Moreover,a fourth aspect of the present invention is an illumination devicecharacterized by including the semiconductor light-emitting elementaccording to the above-described third aspect.

Moreover, the fourth aspect of the present invention is a sputteringapparatus comprising: a target electrode on which a target can bearranged; a substrate holder on which a substrate can be arranged towardthe target electrode and provided with a heater electrode and a biaselectrode; and frequency interference suppressing means for preventingfrequency interference from occurring between the high-frequency powerapplied to the target electrode and the high-frequency bias powerapplied to the bias electrode when the step of forming an epitaxial filmof the group III nitride semiconductor thin film according to theabove-described first aspect is performed.

According to the present invention, an epitaxial film of a group IIInitride semiconductor in which mosaic expansion of tilt or twist issmall, mixture of +c polarity and −c polarity is reduced, andunification of the +c polarity is improved can be fabricated on theα-Al₂O₃ substrate by using the sputtering method. Moreover, by using thegroup III nitride semiconductor epitaxial film fabricated by thissputtering method, light emission characteristics of light-emittingelements such as LED and LD can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional schematic diagram of a high-frequency sputteringapparatus according to one embodiment of the present invention.

FIG. 2 is a first configuration example of a substrate holder accordingto one embodiment of the present invention.

FIG. 3 is a second configuration example of a substrate holder accordingto one embodiment of the present invention.

FIG. 4 is a third configuration example of a substrate holder accordingto one embodiment of the present invention.

FIG. 5 is a diagram illustrating a model in which a group III nitridesemiconductor thin film with +c polarity is formed according to oneembodiment of the present invention.

FIG. 6 is a sectional view illustrating an example of an LED structurefabricated by using an epitaxial film formed by an epitaxial filmforming method according to one embodiment of the present invention.

FIG. 7A is a diagram for explaining frequency interference suppressingmeans according to one embodiment of the present invention.

FIG. 7B is a diagram for explaining the frequency interferencesuppressing means according to one embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described below indetail. In the figures which will be described below, the same referencenumerals are given to those having the same functions and duplicatedexplanation will be omitted.

Major features of the present invention is that, when a group IIInitride semiconductor thin film is to be epitaxially grown on an α-Al₂O₃substrate by using a sputtering method such as a high-frequencysputtering method, for example, the α-Al₂O₃ substrate is heated to anarbitrary temperature by a substrate holder provided with a heaterelectrode and a bias electrode and then, a film of the group III nitridesemiconductor is formed while a high-frequency bias power is applied tothe bias electrode. The present invention will be described by referringto the attached drawings. Members, arrangements and the like which willbe described below are one example which embodies the invention and donot limit the present invention and they can be naturally modified invarious ways in accordance with the gist of the present invention.

FIG. 1 is a schematic configuration diagram illustrating an example of asputtering apparatus used in forming a group III nitride semiconductorthin film according to an embodiment of the present invention. In FIG. 1illustrating a sputtering apparatus 1, reference numeral 101 denotes avacuum vessel, reference numeral 102 denotes a target electrode,reference numeral 103 denotes a bias electrode, reference numeral 104denotes a heater electrode, reference numeral 105 denotes a targetshield, reference numeral 106 denotes a high-frequency power supply forsputtering, reference numeral 107 denotes a substrate, reference numeral108 denotes a target, reference numeral. 109 denotes a gas introduction,mechanism, reference numeral 110 denotes an evacuation mechanism,reference numeral 111 denotes a substrate holder, reference numeral 112denotes a reflector, reference numeral 113 denotes an insulatingmaterial, reference numeral 114 denotes a chamber shield, referencenumeral 115 denotes a magnet unit, reference numeral 116 denotes atarget-shield holding mechanism, and reference numeral 130 denotes ahigh-frequency power supply for bias, respectively. Reference numeral111 in FIG. 1 is assumed to be arbitrary one of substrate holders 111 a,111 b, and 111 c which will be described later. Moreover, as thesubstrate 107, an α-Al₂O₃ substrate (601) can be used.

The vacuum vessel 101 is formed of a metal member such as stainless andaluminum alloy and is electrically grounded. Moreover, the vacuum vessel101 prevents or reduces temperature rise of a wall surface by a coolingmechanism, not shown. Moreover, the vacuum vessel 101 is connected tothe gas introduction mechanism 109 through a mass flow controller, notshown, and is connected to the evacuation mechanism 110 through avariable conductance valve, not shown.

The target shield 105 is mounted on the vacuum vessel 101 through thetarget-shield holding mechanism 116. The target-shield holding mechanism116 and the target shield 105 can be made of a metal member such asstainless and aluminum alloy and is electrically connected to the vacuumvessel 101.

The target electrode 102 is mounted on the vacuum vessel 101 through theinsulating material 113. Moreover, the target 108 is mounted on thetarget electrode 102, and the target electrode 102 is connected to thehigh-frequency power supply 106 for sputtering through a matching box,not shown. The target 108 may be directly mounted on the targetelectrode 102 or may be mounted on the target electrode 102 through abonding plate, not shown, made of a metal member such as copper (Cu).Moreover, the target 108 may be a metal target containing at least oneof Al, Ga, or In or a nitride target containing at least, one of theabove-described group III elements. On the target electrode 102, acooling mechanism, not shown, for preventing temperature rise of thetarget 108 is provided. Moreover, the target electrode 102 incorporatesthe magnet unit 115, As power to be input to the target electrode 102from the high-frequency power supply 106 for sputtering, industrially13.56 MHz can be used easily, but it is also possible to use highfrequency of another frequency, to superimpose a direct current on ahigh frequency or to use them in pulse.

The chamber shield 114 is mounted on the vacuum vessel 101 and preventsor reduces adhesion of a film to the vacuum vessel 101 during filmformation. The substrate holder 111 has the heater electrode 104 and thebias electrode 103 therein. To the heater electrode 104, a power supplyfor heating, not shown, is connected, while to the bias electrode 103, ahigh-frequency power supply 130 for bias is connected through a matchingbox, not shown.

FIGS. 2 to 4 illustrate the configuration examples 111 a, 111 b, and 111c of the substrate holder 111, respectively, and reference numeral M ineach figure denotes a substrate placement surface. In FIG. 2 (or FIG.3), reference numeral 201 denotes a base, reference numeral 202 denotesa base coat, reference numeral 103 a (or reference numeral 103 b)denotes a bias electrode, reference numeral 104 denotes the heaterelectrode, and reference numeral 205 denotes an over coat. The base 201is made of graphite, the base coat 202 and the over coat 205 are made ofpyrolytic boron nitride (PBN), and the bias electrode 103 a (or 103 b)and the heater electrode 104 are made of pyrolvtic graphite (PG), andthe base coat 202 and the over coat 205 made of PBN are dielectricmaterials with high resistance.

In FIG. 2 (or FIG. 3), a power supply for heating, not shown, isconnected to the heater electrode 104. Joule heat is generated by havingan AC or DC current flow through the heater electrode 104, and theα-Al₂O₃ substrate placed on the substrate holder 111 a (or 111 b) can beheated by infrared rays from the substrate holder heated by the Jouleheat.

Moreover, in FIG. 2 (or FIG. 3), the high-frequency power supply 130 forbias is connected to the bias electrode 103 a (or 103 b) through amatching box, not shown. By applying high-frequency bias power to thebias electrode 103 a (or 103 b) during film formation, a negative DCbias voltage can be generated, on the surface of the α-Al₂O₃ substrateplaced on the substrate holder 111 a (or 111 b).

It is also possible to further connect a power supply for anelectrostatic chuck (ESC), not shown, to the bias electrode 103 a (or103 b) in FIG. 2 (or FIG. 3) through a low-pass filter, not shown. Insuch a case, the bias electrode 103 a is configured as electricallyseparated electrodes as indicated by reference numeral A and referencenumeral B (one is a first electrode, while the other is a secondelectrode), and a bipolar ESC may be realized by applying positive andnegative DC voltages to the electrodes, respectively. In this way, sincethe α-Al₂O₃ substrate can be electrostatically adsorbed to the substrateholder, the α-Al₂O₃ substrate placed on the substrate holder 111 a canbe efficiently heated. With regard to the subscrate holder 111 b, too,the bias electrode 103 b is configured as electrically separatedelectrodes as indicated by reference numeral C and reference numeral D(one is a first electrode, while the other is a second electrode) and abipolar ESC can be realized by applying positive and negative DCvoltages to the electrodes, respectively.

FIG. 4 is another configuration example 111 c of the substrate holder111. Reference numeral 401 denotes a base, reference numeral 402 denotesa base coat, reference numeral 403 denotes a common electrode, referencenumeral 404 denotes a backside coat, and reference numeral 405 denotesan over coat. The base 401 is made of graphite, the base coat 402 andthe over coat 405 are made of PBN, the common electrode 403 and thebackside coat 404 are made of PG, and the base coat 402 and the overcoat 405 made of PBN are dielectric materials with high resistance.

In FIG. 4, to the common electrode 403, the high-frequency power supply130 for bias is connected through a matching box, and moreover, a powersupply for heating, not shown, is connected through a low-pass filter,not shown.

In FIG. 4, the common electrode 403 has a function integrating theheater electrode 104 and the bias electrode 103 a in FIG. 2. By havingan AC or DC current flow from the power supply for heating to the commonelectrode 403, the substrate holder 111 c produces heat, and the α-Al₂O₃substrate placed on the substrate holder 111 c is heated by the infraredrays thereof. Moreover, by further applying high-frequency bias powerfrom the high-frequency power supply for bias in a state where thecurrent for heating is made to flow through the common electrode 403, anegative DC bias voltage can be generated on the surface of the α-Al₂O₃substrate, while the α-Al₂O₃ substrate placed on the substrate holder111 c is heated. In this way, the effect of the present invention can bealso obtained by using the common electrode integrating the heaterelectrode and the bias electrode into one.

In the substrate holder 111 a having a structure illustrated in FIG. 2,Joule heat generated from the heater electrode 104 is transmitted to thesubstrate placement surface M side through the base coat 202, the base201, the over coat 205, and the bias electrode 103 a. At this time,particularly since the base 201 plays a role as a soaking plate, highsoaking performance can be easily obtained in the substrate plane.

Moreover, in the substrate holder 111 b having a structure illustratedin FIG. 3, the bias electrode 103 b is formed of a substantiallydisk-shaped electrode (corresponding to reference numeral C) at thecenter part and a substantially ring-shaped electrode (corresponding toreference numeral D) on an outer periphery part. Thus, the biaselectrode 103 b (particularly the reference numeral C part) furtherplays a role of a soaking plate and can further improve the soakingperformance in the plane more than the substrate holder 111 a having thestructure illustrated in FIG. 2. Particularly, if the α-Al₂O₃ substrateis adsorbed by the ESC, temperature distribution depending on a patternshape of the bias electrode 103 a might be generated in the substrateholder 111 a having the structure illustrated in FIG. 2, but with thestructure as in FIG. 3, such problem can be markedly improved.

By using the ESC, a temperature rising speed after the α-Al₂O₃ substrateis placed on the substrate holders 111 a and 111 b can be increased andthus, use of the ESC is a preferable mode in obtaining highproductivity.

In the substrate holder 111 c having a structure illustrated in FIG. 4,Joule heat generated from the common electrode 403 is transmitted to thesubstrate placement surface M side not through the base 401 or the basecoat 402. Thus, as compared with the substrate holders illustrated inFIGS. 2 and 3, it is difficult to obtain high soaking performance. Onthe other hand, since the α-Al₂O₃ substrate is heated not though thebase 401 or the base coat 402, temperature gradient of the substrateplacement surface M and the common electrode 403 becomes small, and thesubstrate can be heated with high power use efficiency without using theESC.

The material constituting the substrate holder illustrated in FIGS. 2 to4 is preferably used since efficiency to heat the α-Al₂O₃ substrate ishigher than a prior-art infrared lamp, but this is not limiting as longas the α-Al₂O₃ substrate can be heated to a predetermined temperature.

Moreover, the substrate holder is not limited to the structures of theabove-described substrate holders 111 a, 111 b, and 111 c. Thestructures of the above-described substrate holders 111 a, 111 b, and111 c are preferable modes since they can improve soaking performanceand power use efficiency, and thus the structure can be selected asappropriate in accordance with the purpose. However, in the presentinvention, it is important that a negative DC bias voltage is generatedon the surface of the α-Al₂O₃ substrate by applying high-frequency biaspower to the bias electrode at a predetermined temperature, and as aresult, an epitaxial film of the group III nitride semiconductor can beformed with +c polarity. Therefore, it is needless to say that anystructure can be applied to this embodiment, as long as the structureaccords with the gist of the present invention.

FIG. 5 is a model, diagram illustrating a mechanism in which the groupIII nitride semiconductor thin film is formed with +c polarity byapplying high-frequency bias power to the bias electrode. In FIG. 5,reference numeral 111 denotes an arbitrary substrate holder in thesubstrate holders 111 a, 111 b, and 111 c, and reference numeral 107 isan α-Al₂O₃ substrate, and reference numeral 503 is a nitride molecule.

FIG. 6 is an example of a sectional structure of a light-emitting diode(LED) as a semiconductor light-emitting element fabricated by using themanufacturing method of the group III nitride semiconductor thin filmaccording to the embodiment of the present invention. In FIG. 6,reference numeral 601 denotes an α-Al₂O₃ substrate, reference numeral602 denotes a buffer layer, reference numeral 603 denotes a group IIInitride semiconductor intermediate layer, reference numeral 604 denotesan n-type group III nitride semiconductor layer, reference numeral 605denotes a group III nitride semiconductor active layer, referencenumeral 606 denotes a p-type group III nitride semiconductor layer,reference numeral 607 denotes a translucent electrode, reference numeral608 denotes an n-type electrode, reference numeral 609 denotes a p-typebonding pad electrode, and reference numeral 610 denotes a protectivefilm.

As a material constituting the buffer layer 602, AlN, AlGaN, and GaN arepreferably used. As a material constituting the group III nitridesemiconductor intermediate layer 603, the n-type group III nitridesemiconductor layer 604, the group III nitride semiconductor activelayer 605, and the p-type group III nitride semiconductor layer 606,AlGaN, GaN and InGaN are preferably used. It is preferable that silicon(Si) or germanium (Ge) is added m an extremely small amount in theabove-described material for the n-type group III nitride semiconductorlayer 604, and magnesium (Mg) or zinc (Zn) is added in an extremelysmall amount in the above-described material for the p-type group IIInitride semiconductor layer 606, respectively, for executing control ofconductivity. Moreover, the group III nitride semiconductor active layer605 preferably forms a multiple quantum well (MQW) structure of theabove-described materials. Furthermore, an illumination device can beconfigured by using the above-described light-emitting diode (LED).

FIGS. 7A and 7B are diagrams for explaining frequency interferencesuppressing means according to the embodiment of the present invention.

FIG. 7A is an example of means for suppressing frequency interference(frequency interference suppressing means) which will be described laterby using high-frequency power supplies at different frequencies as thehigh-frequency power supply 106 for sputtering and the high-frequencypower supply 130 for bias. Reference numerals 701 and 702 illustratematching boxes. The high-frequency power from the high-frequency powersupply 106 for sputtering is supplied to the target electrode 102 with areflective wave being reduced by going through the matching box 701,while the high-frequency power from the high-frequency power supply 130for bias is supplied to the bias electrode 103 with a reflective wavebeing reduced by going through the matching box 702. The high-frequencypower supply 106 for sputtering and the high-frequency power supply 130for bias are set so as to have different frequencies. For example, ifthe frequency of the high-frequency power supply 106 for sputtering isset to 13.56 MHz, by using a frequency of 13.54 MHz or 13.58 MHz as thehigh-frequency power supply 130 for bias, the frequency interferencewhich will be described later can be suppressed.

FIG. 7B illustrates an example of means for suppressing frequencyinterference which will be described later (frequency interferencesuppressing means) by adjusting a phase of the high-frequency power fromthe high-frequency power supply 106 for sputtering and from thehigh-frequency power supply 130 for bias. In FIG. 7B, reference numeral703 denotes a phase control unit, reference numeral 704 denotes ahigh-frequency oscillator, and reference numeral 705 and referencenumeral 706 denote phase adjustment circuits. The high-frequency powerfrom the high-frequency power supply 106 for sputtering is supplied tothe target electrode 102 with a reflective wave being reduced by goingthrough the matching box 701, while the high-frequency power from thehigh-frequency power supply 130 for bias is supplied to the biaselectrode 103 with a reflective wave being reduced by going through thematching box 702.

The phase control unit 703 has the high-frequency oscillator 704 and thephase adjustment circuits 705 and 706, and can adjusts a phase of ahigh-frequency signal from the high-frequency oscillator 704 by thephase adjustment circuits 705 and 706 to output to an external circuit.Moreover, an output portion of the phase control unit 703 is connectedto an external input terminal portion of the high-frequency power supply106 for sputtering and the high-frequency power supply 130 for bias. Bymeans of a high-frequency signal which is output by the phase controlunit 703 and the phase of which is adjusted (that is, a high-frequencysignal oscillated by the high-frequency oscillator 704, and moreover ahigh-frequency signal whose phase is adjusted by the phase adjustmentcircuits 705 and 706), a phase of the high-frequency power output fromthe high-frequency power supply 106 for sputtering and thehigh-frequency power supply 130 for bias is controlled. For example, byadjusting the phase control unit 703 and by setting a phase differenceof the high-frequency power output from the high-frequency power supply106 for sputtering and the high-frequency power supply 130 for bias to aphase difference of 180° or the like, frequency interference which willbe described later can be suppressed.

In order to prevent frequency interference which will be described laterfrom occurring as described above, setting the high-frequency powersupplied to the target electrode 102 and the high-frequency powersupplied to the bias electrode 103 so as to have different frequenciesor keeping the high-frequency power supplied to the target electrode 102and the high-frequency power supplied to the bias electrode 103 at apredetermined phase difference are effective means. In order to obtainthe effect of the present invention with high reproducibility, provisionof these frequency interference suppressing means is extremelyeffective.

An epitaxial film forming method of a group III nitride semiconductorthin film by using the sputtering apparatus according to the presentinvention will be described below by referring to the attached drawings.In this embodiment, an epitaxial film is formed on an α-Al₂O₃ substrateby a method having the following first to fourth steps. In the followingdescription, the substrate holder 111 is assumed to refer to anarbitrary one of the substrate holders 111 a, 111 b, and 111 c, and thebias electrode 103 is assumed to refer to the bias electrodes 103 a, 103b, and 403 (403 is a common electrode) provided in the arbitrarysubstrate holders 111 (111 a, 111 b, and 111 c).

First, as the first step, the substrate 107 is introduced into thevacuum vessel 101 maintained at a predetermined pressure by theevacuation mechanism 110. At this time, the substrate (α-Al₂O₃substrate) 107 is conveyed to the upper part of the substrate holder 111by a conveyer robot, not shown, and is held at an upper part of a liftpin, not shown, protruding from the substrate holder 111. After that,the lift pin holding the substrate 107 is lowered, and the substrate 107is placed on the substrate holder 111.

Subsequently, as the second step, a voltage to be applied to the heaterelectrode 104 incorporated in the substrate holder 111 is controlled soas to maintain the substrate 107 at a predetermined temperature. At thistime, control is executed such that the temperature of the substrateholder 111 is monitored by using a thermocouple (not shown) incorporatedin the substrate holder 111 or the temperature of the substrate holder111 is monitored by using a pyrometer, not shown, installed in thevacuum vessel 101 so that the temperatures become predeterminedtemperatures.

Subsequently, as the third step, either of an N₂ gas or a mixed gas ofan N₂ gas and a rare gas is introduced into the vacuum vessel 101 fromthe gas introduction mechanism 109, and the pressure of the vacuumvessel 101 is set to a predetermined pressure by a mass flow controller(not shown) and a variable conductance valve (not shown).

Lastly, as the fourth step, by applying a high-frequency bias power tothe bias electrode 103 incorporated in the substrate holder 111 and byapplying a high-frequency power to the target 108 by the high-frequencypower supply 106 for sputtering, plasma is generated on a front surfaceof the target 108. As a result, an ion in the plasma throws out elementsconstituting the target 108, and a group III nitride semiconductor thinfilm is formed on the substrate 107 by the thrown out elements.

The predetermined pressure in the first step is preferably less than5×10⁻⁴ Pa and if the pressure is not smaller than that, many impuritiessuch as oxygen and the like are taken into the group III nitridesemiconductor thin film, and it is difficult to obtain a favorableepitaxial film. Moreover, the temperature of the substrate holder 111 inthe first step is not particularly limited, but from the viewpoint ofproductivity, a temperature for obtaining a substrate temperature duringfilm forming is preferably set.

The predetermined temperature in the second step is preferably set to afilm forming temperature in the fourth step from the viewpoint ofproductivity, and the predetermined pressure in the third step ispreferably set to a film forming pressure in the fourth step from theviewpoint of productivity. Operation timings of the second step and thethird step may be exchanged or they may be performed at the same time.Moreover, the temperature set in the second step and the pressure set inthe third step are preferably maintained at least until the fourth stepis started from the viewpoint of productivity.

In the fourth step, the high-frequency bias power to be applied to thebias electrode 103 needs to be set to predetermined power with which agroup III nitride semiconductor film (a group III nitride semiconductorthin film with +c polarity) with high unification degree of +c polaritycan be obtained. If power is too small, a group III nitridesemiconductor thin film in which polarities are mixed is formed, whileif power is too large, the group III nitride semiconductor thin film isdamaged by collision with high-energy particles, and a good qualitygroup III nitride semiconductor thin film cannot be obtained.

In this description, a group III nitride semiconductor thin film inwhich −c polarity is not present or reduced, that is, a group IIInitride semiconductor thin film in which mixture of the +c polarity andthe −c polarity is reduced, and a unification degree of the +c polarityis high is called “a group III nitride semiconductor thin film with +cpolarity”.

Moreover, the substrate temperature when the fourth step is performed ispreferably set within a range of 100 to 1200° C., or more preferablywithin a range of 400 to 1000° C. If it is less than 100° C., a film inwhich an amorphous structure is mixed is formed easily, while if it ishigher than 1200° C., a film itself is not formed or even if a film isformed, an epitaxial film with many defects is obtained easily due toheat stress. Moreover, a film forming pressure is preferably set withina range of 0.1 to 100 mTorr (1.33×10⁻² to 1.33×10¹ Pa), or morepreferably set within a range of 0.1 to 10 mTorr (1.33×10⁻¹ to 1.33 Pa).

If it is less than 0.1 mTorr (1.33×10⁻² Pa), high energy particles caneasily enter the substrate surface, and thus it is difficult to obtain agood quality group III nitride semiconductor thin film, while if thepressure is higher than 100 mTorr (1.33×10¹ Pa), a film-forming speedbecomes extremely low, which is not preferable. When the fourth step isto be started, it is possible to temporarily raise the pressure of thevacuum vessel 101 to the film-forming pressure or more so as to promotegeneration of plasma on the target side. In this case, the film-formingpressure may be raised by temporarily introducing a large gas flow rateof at least one type of process gases or by temporarily decreasing anopening degree of the variable conductance valve (not shown).

In the fourth step, the timing to apply high-frequency bias power to thebias electrode 103 and the timing to apply high-frequency power to thetarget electrode 102 may be the same or may be set such that one isapplied first and then, the other is applied. However, if thehigh-frequency power is to be applied to the target electrode 102,first, the high-frequency bias power needs to be applied to the biaselectrode 103 before a film formation surface of the α-Al₂O₃ substrate107 is covered by a crystal layer made of a group III nitridesemiconductor.

The crystal layer of the group III nitride semiconductor formed in astate where the high-frequency bias power is not applied to the biaselectrode 103 is easily brought into a state in which polarities aremixed or a state of −c polarity. If a state in which the −c polarity ismixed occurs, it is difficult to obtain the group III nitridesemiconductor thin film with +c polarity even if the high-frequency biaspower is applied to the bias electrode 103 after that. Therefore, if thehigh-frequency power is to be applied to the target electrode 102,first, it is preferable that the high-frequency power is applied to thetarget electrode 102 and immediately after plasma is generated on thefront surface of the target (that is, after sputtering is started), thehigh-frequency bias power is applied to the bias electrode 103 andbefore a crystal layer made of the group III nitride semiconductor isformed on the α-Al₂O₃ substrate 107, the high-frequency bias power isapplied to the bias electrode 103.

If the high-frequency bias power is to be applied to the bias electrode103, first, plasma is generated on the surface side of the α-Al₂O₃substrate 107 during a period until the high-frequency power is appliedto the target electrode 102, and surface nitridization of the α-Al₂O₃substrate 107 caused by an active species containing an N atom in theplasma needs to be avoided. That is because, if the surface of theα-Al₂O₃ substrate 107 is nitrided, AlN with −c polarity or with mixedpolarity is easily formed on the substrate surface and even if thehigh-frequency power is applied to the target, electrode 102 after thatso as to form the group III nitride semiconductor thin film, it becomesdifficult to obtain a group III nitride semiconductor thin film with +cpolarity. Therefore, if the high-frequency bias power is to be appliedto the bias electrode 103, first, it is preferable that thehigh-frequency power is applied to the target electrode 102 immediatelyafter the high-frequency bias power is applied to the bias electrode 103and then, sputtering is started.

Moreover, it is needless to say that before the first step, such a stepmay be provided in which the substrate 107 is conveyed to apre-treatment chamber (not shown) for thermal treatment or plasmaprocessing of the substrate 107 at a temperature of the film-formingtemperature or more. However, if the plasma processing is to beperformed, it is important to select a condition that an AlN layer withmixed polarity or an AlN layer with −c polarity is not formed on thesurface of the α-Al₂O₃ substrate.

A mechanism of forming a group III nitride semiconductor thin film with+c polarity by the above-described first to fourth steps will bedescribed below by using FIG. 5. As the first and second steps, theα-Al₂O₃ substrate 107 is placed on the substrate holder 111 so that thesubstrate is at a predetermined temperature, and as the third step,either of an N₂ gas or a mixed gas of an N₂ gas and a rare gas isintroduced into the vacuum vessel. Subsequently, as the fourth step,high-frequency bias power is applied to the bias electrode, and alsoplasma is generated on the target side so as to form a group III nitridesemiconductor thin film.

If a metal target is used as a target in the fourth step, it isconsidered that the target surface is nitrided by an active speciescontaining an N atom, and by sputtering the surface with a positive ion,a nitride molecule 503 illustrated in FIG. 5 is emitted from the targetsurface to reach the surface of the α-Al₂O₃ substrate 107. Moreover,even if a nitride target is used, it is considered that by sputteringthe surface with a positive ion, a nitride molecule 503 illustrated inFIG. 5 is emitted from the target surface to reach the surface of theα-Al₂O₃ substrate 107. A nitride molecule 503 of a 2-atom molecule isillustrated in FIG. 5 for simplification, but the molecule is notlimited to a 2-atom molecule as long as it is a nitride molecule.

In FIG. 5, high-frequency bias power is applied to the bias electrode103, and in a space facing the surface side of the α-Al₂O₃ substrate107, a plasma region indicated by reference numeral G and a sheathregion indicated by reference numeral S are formed. The sheath region Sis formed between the plasma region G and the α-Al₂O₃ substrate 107.

In the plasma region G, densities of a positive charge (positive ion)and a negative charge (electron) are substantially equal andsubstantially in an electrically neutral state. Moreover, the plasmaregion G is usually in a substantially constant potential state (calledplasma potential) which is positive with respect to grounding potential.On the other hand, since a positive ion and an electron are different inease of follow-up with respect to a change in a high-frequency electricfield generated by application of the high-frequency bias power,excessive electrons are supplied to the surface of the α-Al₂O₃ substrate107, and a negative DC bias voltage is generated. In the sheath regionS, an electric field indicated by reference numeral E in a directiontoward the surface of the α-Al₂O₃ substrate 107 is generated by apotential difference between the negative potential on the surface ofthe α-Al₂O₃ substrate 107 generated as above and the plasma potential ofthe plasma region G. The size of this electric field E can be adjustedby magnitude of the high-frequency bias power.

As a mode of power to be applied to the bias electrode 103,high-frequency power is preferable. In the case of DC power, since theα-Al₂O₃ substrate 107 is an insulating material, it becomes difficult toeffectively generate a negative DC bias voltage on the surface of theα-Al₂O₃ substrate 107, which is not preferable.

The nitride molecule 503 has a group III element 503 a and an N atom 503b, and the group III element 503 a has positive charge biased and the Natom 503 b has negative charge biased. That is, the nitride molecule 503has polarization indicated by reference numeral P, This nitride molecule503 is considered to be oriented in random directions in the plasmaregion G, but when it reaches the sheath region S, the electric field Eacts on the polarization P of the nitride molecule 503, and it isconsidered that the group III element 503 a is oriented in the directionof the α-Al₂O₃ substrate and the N atom 503 b is oriented in thedirection of the plasma region G, that is, the polarization P isoriented to the direction of the α-Al₂O₃ substrate.

In the group III nitride semiconductor with +c polarity, thepolarization P of the nitride molecule 503 is oriented to the directionof the α-Al₂O₃ substrate. That is, it is considered that thepolarization P of the nitride molecule 503 is oriented to the directionof the α-Al₂O₃ substrate by the electric field E of the sheath region Sgenerated by application of the high-frequency bias power, and by meansof adsorption to the α-Al₂O₃ substrate surface while the orientation ismaintained, a group III nitride semiconductor thin film with +c polarityIs obtained.

Even if the high-frequency bias power is applied to the bias electrode103, if the high-frequency bias power is small, a group III nitridesemiconductor with +c polarity might not be obtained. The reason forthat is considered to be that the electric field E of the sheath regionS did not sufficiently act on the polarization P of the nitride molecule503 and could not control orientation.

Moreover, if the high-frequency bias power is too large, a high-qualitygroup III nitride semiconductor might not be obtained. The reason forthat is considered to be that the positive ion in the plasma isaccelerated by the electric field E in the sheath region S and collidesagainst the surface of the α-Al₂O₃ substrate with large energy, and thusmany defects are formed inside the group III nitride semiconductor thinfilm.

As described above, in order to obtain a group III nitride semiconductorthin film with +c polarity, it is necessary to adjust the magnitude ofthe high-frequency bias power to be applied to the bias electrode 103 toan appropriate value. An optimal range of this high-frequency bias poweris different largely depending on the internal structure of thesputtering apparatus, and thus an optimal condition needs to be acquiredfor each apparatus.

Moreover, a frequency used as the high-frequency bias power is notparticularly limited, but if the frequency of the high-frequency biaspower and the frequency of the high-frequency power applied to thetarget match each other, a low-frequency beat phenomenon caused byinterference of high-frequency power may easily occur, and afilm-forming condition might be also affected (hereinafter thislow-frequency beat phenomenon is called frequency interference). Iffrequency interference occurs in this embodiment, plasma becomesunstable and the DC bias voltage generated on the surface of the α-Al₂O₃substrate no longer becomes stable, and thus high frequency power at adifferent frequency is preferably used. By using FIG. 1A as an example,assuming that a frequency of high-frequency power to be applied to thetarget electrode 102 (a frequency of the high-frequency power supply 106for sputtering) is 13.56 MHz, by using 13.54 MHz or 13.58 MHz for thefrequency of the high-frequency bias power (frequency of thehigh-frequency power supply 130 for bias) to be applied to the biaselectrode 103, the above-described frequency interference can beprevented or reduced.

Moreover, by shifting the high-frequency bias power to be applied to thebias electrode and the high-frequency power to be applied to the targetby a predetermined phase difference, the above-described frequencyinterference can be suppressed. By using FIG. 7B as an example, if aphase difference between the high-frequency bias power to be applied tothe bias electrode 103 and the high-frequency power to be applied to thetarget electrode 102 is adjusted to be 180° by the phase control, unit703, that is, if adjustment is made so that a positive peak top voltageof the high-frequency power is applied to the target electrode 102 andat the same time, a negative peak top voltage of the high-frequency biaspower is applied to the bias electrode 103, frequency interference canbe prevented or reduced most effectively. Moreover, the phase differencemay be finely adjusted so that a reflective wave to each of thehigh-frequency power supplies (high-frequency power-supply forsputtering and high-frequency power supply for bias) is further reduced.That is, it is assumed that the phase difference of 180° includes arange of fine adjustment.

Moreover, even in the case of other phase difference, it can be usedwithout problem unless frequency interference is caused. If frequencyinterference as above occurs, the plasma becomes unstable and areflective wave to each of the high-frequency power supplies(high-frequency power supply for sputtering and high-frequency powersupply for bias) may easily increase, and thus it is preferable thatadjustment is made to such a phase difference that minimizes thefrequency interference (preferably to zero).

Even if the high-frequency bias power is not applied to the biaselectrode 103, the electric field E is generated in the sheath region S,but the electric field E generated at this time is generally smallerthan that generated when the high-frequency bias power is applied.Therefore, the reason why a nitride semiconductor thin film with +cpolarity cannot, be obtained if the high-frequency bias power is notapplied to the bias electrode 103 is considered to be that the electricfield E of the sheath region S does not sufficiently act on thepolarization P of the nitride molecule 503 and cannot controlorientation.

When the metal target 108 is to be sputtered by plasma using a mixed gasof an N₂ gas and a rare gas, a ratio in the mixed gas of the N₂ gas andthe rare gas should be controlled so that a metal component (non-nitridecomponent) is not taken in the group III nitride semiconductor thin filmin a large amount. If a large amount of the metal component is taken in,the ratio of a metal atom or the group III element emitted from thetarget in a metal cluster state tends to become larger than the nitridemolecule 503, and thus it is likely that the effect of the presentinvention cannot be fully obtained even if the high-frequency bias poweris applied to the bias electrode 103.

As the epitaxial film of the group III nitride semiconductor thin filmformed by the method in this embodiment, the buffer layer 602, the groupIII nitride semiconductor intermediate layer 603, the n-type group IIInitride semiconductor layer 604, the group III nitride semiconductoractive layer 605, and the p-type group III nitride semiconductor layer606 illustrated in FIG. 6 can be cited. All of the above-describedlayers may be fabricated by using the sputtering apparatus (epitaxialfilm forming method) according to the present invention or only limitedlayers may be fabricated by using the sputtering apparatus (epitaxialfilm forming method) according to the present invention.

As a first example, for example, there is a method of fabricating anepitaxial wafer by fabricating the buffer layer 602 of an LED element inFIG. 6 by using the sputtering apparatus (epitaxial film forming method)according to the present invention and then, by sequentially laminatingthe group III nitride semiconductor intermediate layer 603, the n-typegroup III nitride semiconductor layer 604, the group III nitridesemiconductor active layer 605, and the p-type group III nitridesemiconductor layer 606 by using the MOCVD method.

As a second example, there is a method of fabricating an epitaxial waferby fabricating the buffer layer 602 and the group III nitridesemiconductor intermediate layer 603 by using the sputtering apparatus(epitaxial film forming method) according to the present invention andthen, by sequentially laminating the n-type group III nitridesemiconductor layer 604, the group III nitride semiconductor activelayer 605, and the p-type group III nitride semiconductor layer 606 byusing the MOCVD method.

As a third example, there is a method of fabricating an epitaxial waferby fabricating the buffer layer 602, the group III nitride semiconductorintermediate layer 603, and the n-type group III nitride semiconductorlayer 604 by using the sputtering apparatus (epitaxial film formingmethod) according to the present invention and then, by sequentiallylaminating the group III nitride semiconductor active layer 605 and thep-type group III nitride semiconductor layer 606 by using the MOCVDmethod.

As a fourth example, there is a method of fabricating an epitaxial waferby fabricating the buffer layer 602, the group III nitride semiconductorintermediate layer 603, the n-type group III nitride semiconductor layer604, and the group III nitride semiconductor active layer 605 by usingthe sputtering apparatus (epitaxial film forming method) according tothe present invention and then, by fabricating the p-type group IIInitride semiconductor layer 606 by using the MOCVD method.

As a fifth example, there is a method of fabricating an epitaxial waferby fabricating the buffer layer 602, the group III nitride semiconductorintermediate layer 603, the n-type group III nitride semiconductor layer604, the group III nitride semiconductor active layer 605, and thep-type group III nitride semiconductor layer 606 by using the sputteringapparatus (epitaxial film forming method) according to the presentinvention.

On the epitaxial wafer obtained as above, by forming the translucentelectrode 607, the p-type bonding pad electrode 609, the n-typeelectrode 608, and the protective film 610 as illustrated in FIG. 6 byusing the lithography technology and RIE (reactive ion etching)technology, an LED structure can be obtained. The material for thetranslucent electrode 607, the p-type bonding pad electrode 609, then-type electrode 608, and the protective film 610 is not particularlylimited but materials well-known in this technical field can be usedwithout limitation.

EXAMPLE First Example

As a first example of the present invention, an example in which an AlNfilm as the buffer layer 602 (See FIG. 6) was formed on an α-Al₂O₃(0001) substrate by using the film-forming method of a group III nitridesemiconductor thin film according to an embodiment of the presentinvention will be described. More specifically, an example in which, ina state where the high-frequency bias power was applied to the biaselectrode 103, the AlN film was formed on the α-Al₂O₃ (0001) substrateby using the sputtering method will be described. In the first example,the AlN film was formed by using the sputtering apparatus similar tothat in FIG. 1. Moreover, the frequencies of the high-frequency power tobe applied to the target electrode 102 and the high-frequency power tobe applied to the bias electrode 103 are set to 13.56 MHz and 13.54 MHz,respectively.

In the first example, first, the α-Al₂O₃ (0001) substrate was conveyedto the vacuum vessel 101 maintained at 1×10⁻⁴ Pa or less and placed onthe substrate holder 111 by the first step, and the substrate wasmaintained at 550° C. which was a film-forming temperature by the secondstep. At this time, a current made to flow to the heater electrode 104was controlled so that a monitor value of the thermocouple incorporatedin the substrate holder 111 was at 750° C.

Subsequently, by the third step, a mixed gas of an N₂ gas and an Ar gaswas introduced so that N₂/(N₂+Ar): 25% was realized, and the pressure ofthe vacuum vessel 101 was set to 3.75 mTorr (0.5 Pa). In this state, thehigh-frequency bias power at 10 W was applied to the bias electrode 103by the fourth step, and also the high-frequency power at 2000 W wasapplied from the high-frequency power supply 106 for sputtering to thetarget 108 made of metal Al, and an AlN film having a film thickness of50 nm was formed on the substrate by the sputtering method. It wasconfirmed by X-ray photoelectron spectroscopy (XPS) at this time thatthe obtained AlN film contained almost no metal Al component.

The film-forming temperature in the first example was set from arelationship between the temperature of the α-Al₂O₃ (0001) substrate andthe monitor value of the thermocouple incorporated in the heater at thattime, that is, the temperature of the heater by conducting substratetemperature measurement in advance of the α-Al₂O₃ (0001) substrate inwhich the thermocouple was embedded.

In the first example, the fabricated AlN film was evaluated by X-raydiffraction (XRD) measurement in a 2θ/ω scan mode at a symmetricreflection position, XRC measurement in a ω scan mode for the symmetricplane, the XRC measurement in a ϕ scan mode in In-plane arrangement, andcoaxial impact collision ion scattering spectroscopy (CAICISS)measurement. Here, the XRD measurement in the 2θ/ω scan mode at: asymmetric reflection position was used for checking crystal orientation,and the XRC measurement in the scan mode for the symmetric plane and theXRC measurement in the ϕ scan mode in In-plane arrangement were used forevaluation of mosaic expansion of tilt and twist, respectively.Moreover, the CAICISS measurement was used as means for determiningpolarity.

First, the XRD measurement in the 2θ/ω scan mode at a symmetricreflection position was conducted for the AlN film fabricated in thefirst example using a measurement range of 20=20 to 60°, onlydiffraction peaks of the AlN (0002) plane and the α-Al₂O₃ (0006) planewere observed, and a diffraction peak indicating other lattice planes ofAlN was not observed. From this fact, it was known that the obtained AlNfilm had c-axis orientation.

Subsequently, the XRC measurement in the to scan mode for the symmetricplane (AlN (0002) plane in the first example) was conducted for the AlNfilm fabricated in the first example. The FWHM of the obtained XRCprofile was 450 arcsec or less if a detector was in an open detectorstate and 100 arcsec or less if an analyzer crystal was inserted intothe detector. Thus, it was confirmed that mosaic expansion of tilt inthe fabricated AlN film was small. Moreover, depending on thefabrication conditions, those with the FWHM at 20 arcsec or less in theXRC measurement with the analyzer crystal inserted into the detectorwere also obtained.

The XRC measurement should be conducted by the detector in the opendetector state, but in the case of a sample with a small film thicknessas in the first example, the FWHM of the XRC profile expands due to filmthickness effect or lattice relaxation, and accurate evaluation ofmosaic expansion becomes difficult. Thus, in recent years, the case withthe analyzer crystal inserted into the detector is also handled as theXRC measurement in a wide sense as above. Unless otherwise noted, it isassumed below that the open detector state is used in the XRCmeasurement.

Subsequently, the XRC measurement in the ϕ scan mode in In-planearrangement was made for the AlN film fabricated in the first example.The AlN {10-10} plane was used for the measurement. Six diffractionpeaks appeared at 60° intervals in the obtained XRC profile, and it wasconfirmed that the AlN film had six-time symmetry, that is, the AlN filmepitaxially grew. Moreover, the FWHM acquired from the diffraction peakwith the maximum magnitude was 2.0° or less, and it was known thatmosaic expansion of twist of the fabricated AlN film was relativelysmall. When the α-Al₂O₃ (0001) substrate and the in-plane crystalorientation of the AlN film were compared, it was confirmed that ana-axis of the AlN film made in-plane rotation at 30° with respect to thea-axis of the α-Al₂O₃ (0001) substrate. This indicates that the AlN filmwas formed with a general epitaxial relationship when the AlN film wasepitaxially grown on the α-Al₂O₃ (0001) substrate.

Subsequently, for the AlN film fabricated in the first example, theCAICISS measurement was conducted. In this measurement, an Al signal wasdetected with a changed incident angle from an AlN [11-20] orientation,and it was known that a peak in the vicinity of the incident angle of70° was obtained as a single shape. This indicates that the obtained AlNfilm has +c polarity.

As described above, it can be confirmed that the AlN film fabricated inthe first example had +c polarity and is c-axis orientation epitaxialfilm with small mosaic expansion of tilt. That is, according to thepresent invention, it was made clear that a group III nitridesemiconductor thin film with +c polarity could be obtained while mosaicexpansion of tilt and twist was reduced. When the experiment similar tothe first example was repeated several times, it was confirmed thatreproducibility was favorable.

Second Example

Subsequently, as a second example of the present invention, an examplein which the AlN film fabricated by using the film-forming method of thegroup III nitride semiconductor thin film according to the presentinvention was used as a buffer layer and an undoped GaN film as thegroup III nitride semiconductor intermediate layer 603 in FIG. 6 wasformed on the buffer layer by using the MOCVD method will be described.

The AlN film was formed on the α-Al₂O₃ (0001) substrate by using thesputtering method with the same apparatus and conditions as those in thefirst example and then, a wafer (substrate) was introduced into theMOCVD device so as to form an undoped GaN film having a film thicknessof 5 μm.

The surface of the obtained undoped GaN film was a mirror surface, andit was indicated in the XRD measurement in the 2θ/ω scan mode at asymmetric reflection position that the undoped GaN film had a c-axisorientation. Subsequently, the XRC measurement in the ω scan mode usingthe GaN (0002) plane as the symmetric plane and the XRC measurement inthe ϕ scan mode in In-plane arrangement for the GaN {10-10} plane wereconducted, and it was confirmed that the FWHMs were 250 arcsec or lessand 500 arcsec or less, respectively. From this fact, it was known thatthe obtained undoped GaN film was obtained as a high-quality crystalwith small mosaic expansion of tilt and twist. Moreover, from theCAICISS measurement, it was confirmed that the polarity of the obtainedundoped GaN film was −c polarity. This can be considered to be theresult that, as described in the first example, the polarity of the AlNfilm used as the buffer layer can be controlled to the +c polarity andthus, the undoped GaN film formed on the buffer layer also takes overthe polarity.

As described above, by using the AlN film fabricated by using thefilm-forming method of the group III nitride semiconductor thin filmaccording to the present invention and controlled to the +c polarity asthe buffer layer, the undoped GaN film grown by using the MOCVD methodon the buffer layer can be obtained as a high-quality epitaxial filmcontrolled to +c polarity with small mosaic expansion. That is, a groupIII nitride semiconductor thin film with +c polarity can be epitaxiallygrown on the α-Al₂O₃ substrate.

In the second example, the undoped GaN film was formed by the MOCVDmethod, but it was confirmed that the similar result could be obtainedby using the sputtering method, too. Moreover, when the experimentsimilar to the second example was repeated several times, it wasconfirmed that reproducibility was favorable.

Third Example

As a third example of the present invention, an example will bedescribed in which the AlN film fabricated by using the film-formingmethod of the group III nitride semiconductor thin film according to thepresent invention was used as a buffer layer, and on the buffer layer,by using the MOCVD method, the group III nitride semiconductorintermediate layer made of undoped GaN, the n-type group III nitridesemiconductor layer made of Si doped GaN, the group III nitridesemiconductor active layer having an MQW structure of InGaN and GaN, andthe p-type group III nitride semi contractor layer made of Mg doped GaNwere epitaxially grown sequentially and moreover, after the n-typeelectrode layer, the translucent electrode, the p-type electrode layer,and the protective film were formed, a wafer was separated by scribingand an LED element was fabricated.

By using the sputtering method, the AlN film as the buffer layer 602 wasformed on the α-Al₂O₃ (0001) substrate under the same conditions asthose of the first example. After that, the wafer was introduced intothe MOCVD device, and the group III nitride semiconductor intermediatelayer 603 made of undoped GaN having a film thickness of 5 μm and then-type group III nitride semiconductor layer 604 made of Si doped GaNhaving a film thickness of 2 μm were formed. Moreover, the group IIInitride semiconductor active layer 605 having the MQW structure which isa lamination structure starting with GaN and ending with GaN and having5 layers of InGaN with a film thickness of 3 nm and 6 layers of GaN witha film thickness of 16 nm being alternately laminated and the p-typegroup III nitride semiconductor layer 606 made of Mg Doped GaN having afilm thickness of 200 nm were formed.

For the obtained epitaxial wafer, by using the lithography technologyand the RIE technology, the translucent electrode 607, the p-typebonding pad electrode 609, the n-type electrode 608, and the protectivefilm 610 were formed as illustrated in FIG. 6. In the third example, ITO(Indium-Tin-Oxide) was used as the translucent electrode, a structure inwhich Titanium (Ti), Al, and gold (Au) were laminated as the p-typebonding pad electrode, a structure in which nickel (Ni), Al, Ti, and Auwere laminated as the n-type electrode, and SiO₂ as the protective film.

The water on which the LED structure obtained as above was formed wasseparated by scribing into an LED chip of 350 μm square, and this LEDchip was placed on a lead frame and was connected to the lead frame by agold wire so as to have an LED element.

When a forward current was made to flow through the p-type bonding padelectrode and the n-type electrode of the obtained LED element,favorable emission characteristics of a forward voltage at a current of20 mA of 3.0 V, an emission wavelength of 470 nm, and an emission outputof 15 mW were indicated. Such characteristics were obtained withoutvariation for the LED elements fabricated from substantially all thesurfaces of the fabricated wafer.

As described above, by using, as the buffer layer 602, the AlN filmcontrolled to the +c polarity, fabricated by using the film-formingmethod of the group III nitride semiconductor thin film according to thepresent invention, an LED element having favorable emissioncharacteristics could be obtained. In the third example, the group IIInitride semiconductor intermediate layer 603 made of undoped GaN, then-type group III nitride semiconductor layer 604 made of Si doped GaN,the group III nitride semiconductor active layer 605 having the MQWstructure of InGaN and GaN, and the p-type group III nitridesemiconductor layer 606 made of Mg Doped GaN were formed by the MOCVDmethod, but it was confirmed that the similar results could be obtainedby fabricating these layers by using the sputtering method. Moreover,when the experiment similar to the third example was repeated severaltimes, it was confirmed that reproducibility was favorable.

First Comparative Example

As a first comparative example of the present invention, an example inwhich the AlN film was formed on the α-Al₂O₃ (0001) substrate by usingthe sputtering method without applying the high-frequency bias power tothe bias electrode, which is a feature of the present invention, will bedescribed. In the first comparative example, the AlN film was formed byusing the same sputtering apparatus 1, substrate holder 111, andfilm-forming conditions as those in the first example except that thehigh-frequency bias power is not applied to the bias electrode 103.Moreover, the frequency of the high-frequency power to be applied to thetarget electrode 102 is set to 13.56 MHz.

For the AlN film fabricated in the first comparative example, the XRDmeasurement in the 2θ/ω scan mode at a symmetric reflection position,the XRC measurement in the ω scan mode for the AlN (0002) plane (when ananalyzer crystal is inserted in the detector and in an open detectorstate), and the XRC measurement in the ϕ scan mode for AlN {10-10} planewere made, and it was known that an epitaxial film with c-axisorientation was obtained similarly to the AlN film obtained in the firstexample, and mosaic expansion of tilt and twist was at the same degree.On the other hand, when the CAICISS measurement was made for the AlNfilm fabricated in the first comparative example, it was indicated that+c polarity and −c polarity were mixed in the film.

As described above, if a film was formed without applying high-frequencybias power to the bias electrode 103, it was made clear that the groupIII nitride semiconductor thin film with +c polarity could not beobtained. The experiment similar to this comparative example wasrepeated several times, but the AlN film with +c polarity could not beobtained.

Second Comparative Example

Subsequently, as a second comparative example of the present invention,an example in which a buffer layer made of AlN was formed on the α-Al₂O₃(0001) substrate by using the sputtering method without applying thehigh-frequency bias power to the bias electrode 103 and an undoped GaNfilm was formed on the buffer layer by using the MOCVD method will bedescribed. In the second comparative example, the buffer layer made ofAlN was formed by using the same sputtering apparatus 1, substrateholder 111, and film-forming conditions as those in the firstcomparative example, and the undoped GaN film was formed under theconditions similar to those in the second example.

A film of a buffer layer made of AlN was formed on the α-Al₂O₃ (0001)substrate by using the sputtering method using the same sputteringapparatus 1, substrate holder 111, and film-forming conditions as thosein the first comparative example, and after that, the wafer wasintroduced into the MOCVD device, and the undoped GaN film having a filmthickness of 5 μm was formed.

The surface of the obtained undoped GaN film was cloudy, and it wasindicated that in the XRD measurement in the 2θ/ω scan mode at asymmetric reflection position, the undoped GaN film had c-axisorientation. Subsequently, the XRC measurement in the ω scan mode usingthe GaN (0002) plane as a symmetric plane and the XRC measurement, inthe ϕ scan mode for GaN {10-10} plane in the In-plane arrangement weremade, and it was confirmed that the FWHMs were approximately 600 arcsecand 1000 arcsec, respectively. From this fact, it was known that theundoped GaN film obtained by the second comparative example was obtainedas a low-quality crystal with mosaic expansion of tilt and twist largerthan that of the undoped GaN film obtained in the second example.

Moreover, it was confirmed by the CAICISS measurement that the obtainedundoped GaN film was a film in which +c polarity and −c polarity weremixed. This can be considered as the result that, as explained in thefirst comparative example, since the buffer layer made of AlN was a filmin which the +c polarity and the −c polarity were mixed, the undoped GaNfilm formed on the buffer layer took over the mixed polarity thereof.

As described above, if the buffer layer made of AlN is formed on theα-Al₂O₃ (0001) substrate by using the sputtering method without applyingthe high-frequency bias power to the bias electrode, the undoped GaNfilm grown on the buffer layer by using the MOCVD method is obtained asa low-quality epitaxial film. Though the undoped GaN film was formed bythe MOCVD method in the second comparative example, it was confirmedthat the similar result is obtained, too, even when the sputteringmethod is used. Moreover, the experiment similar to this comparativeexample was repeated several times, but a GaN film with a mirror surfaceand favorable crystalline could not be obtained.

Third Comparative Example

As a third comparative example of the present invention, an example inwhich a buffer layer made of AlN was formed on the α-Al₂O₃ (0001)substrate by using the sputtering method without applying thehigh-frequency bias power to the bias electrode and on the buffer layer,the group III nitride semiconductor intermediate layer made of undopedGaN, the n-type group III nitride semiconductor layer made of Si dopedGaN, the group III nitride semiconductor active layer having the MQWstructure of InGaN and GaN, and the p-type group III nitridesemiconductor layer made of Mg Doped GaN were epitaxially grownsequentially by using the MOCVD method and moreover, after the n-typeelectrode layer, the translucent electrode, the p-type electrode layer,and the protective film were formed, the wafer was separated by scribingso as to fabricate an LED element will be described.

The film-forming method of the buffer layer made of AlN is similar tothe first comparative example, and the materials and film-forming methodof the group III nitride semiconductor intermediate layer made ofundoped GaN, the n-type group III nitride semiconductor layer made of Sidoped GaN, the group III nitride semiconductor active layer having theMQW structure of InGaN and GaN, and the p-type group III nitridesemiconductor layer made of Mg doped GaN whose films were formed byusing the MOCVD method and the n-type electrode layer, the translucentelectrode, the p-type electrode layer, and the protective film formedafter that and the step of making an element after that are all similarto those in the third example.

When a forward current was made to flow through the p-type bonding padelectrode and the n-type electrode of the obtained LED element,favorable element characteristics could not be obtained such thatfavorable diode characteristics could not be obtained from the LEDelement, sufficient emission intensity could not be obtained in avisible light region and the like. Such characteristics were obtainedwithout variation for the LED elements fabricated from substantially allthe surfaces of the fabricated wafer.

As described above, it was made clear that an LED element havingfavorable emission characteristics could not be obtained if a bufferlayer made of AlN was formed on the α-Al₂O₃ (0001) substrate by usingthe sputtering method without applying she high-frequency bias power tothe bias electrode. In this example, the group III nitride semiconductorintermediate layer made of undoped GaN, the n-type group III nitridesemiconductor layer made of Si doped GaN, the group III nitridesemiconductor active layer having the MQW structure of InGaN and GaN,and the p-type group III nitride semiconductor layer made of Mg dopedGaN were formed by using the MOCVD method, but it was confirmed that thesimilar result was obtained, too, even if the sputtering method wasused. The experiment similar to this comparative example was repeatedseveral times, but an LED element having favorable emissioncharacteristics could not be obtained.

Fourth Example

As a fourth example of the present invention, an example in whichfrequencies of the high-frequency power to be applied to the targetelectrodes 102 and the high-frequency power to be applied to the biaselectrode 103 were both set to 13.56 MHz, the phase was shifted by 180°,the apparatus and conditions similar to those in the first example wereused for the others, and an AlN film was formed on the α-Al₂O₃ (0001)substrate by using the film-forming method of the group III nitridesemiconductor thin film according to the present invention will bedescribed.

The experiment of the fourth example was conducted repeatedly and it wasconfirmed that an AlN film with +c polarity similar to that in the firstexample could be obtained with good reproducibility.

Fourth Comparative Example

As a fourth comparative example of the present invention, an example inwhich frequencies of the high-frequency power to foe applied to thetarget electrode 102 and the high-frequency power to be applied to thebias electrode 103 were both set to 13.56 MHz, the apparatus andconditions similar to those in the first example were used for theothers, and an AlN film was formed on the α-Al₂O₃ (0001) substrate byusing the film-forming method of the group III nitride semiconductorthin film according to the present invention will be described. In thefourth comparative example, phases of the high-frequency power to beapplied to the target electrode 102 and the high-frequency power to beapplied to the bias electrode 103 were not controlled.

The experiment of the fourth comparative example was conductedrepeatedly and it was made clear that, if frequency interference did notoccur, an AlN film with +c polarity was obtained, but if the frequencyinterference occurred, it became difficult to obtain the AlN film with+c polarity.

As described above about the present invention, the major feature of thepresent invention is that attention is paid to application of thehigh-frequency bias power to the bias electrode in forming an epitaxialfilm of a group III nitride semiconductor by the sputtering method. Theelectric field of the sheath region S generated on the film formationsurface side of the substrate by means of application of thehigh-frequency bias power to the bias electrode is made to act onpolarization of the nitride molecule emitted from the target so as tocontrol orientation, and by using the orientation, a group III nitridesemiconductor thin film with +c polarity can be obtained, which is anunprecedented technical idea.

Moreover, by preventing or reducing low-frequency beat caused byinterference between the high-frequency power applied to the targetelectrode and the high-frequency power applied to the bias electrode,that is, frequency interference, a group III nitride semiconductor thinfilm with + c polarity can be obtained with good reproducibility, whichis an unprecedented technical idea.

In the present invention, under the technical idea specific to thepresent invention, the heater electrode and the bias electrode areprovided on the substrate holder. By configuring the substrate holder asabove, as illustrated in the above-described first to fourth examplesand the first to fourth comparative examples, a group III nitridesemiconductor thin film with reduced mosaic expansion of tilt and twistand having +c polarity can be formed by the sputtering method.

The invention claimed is:
 1. An epitaxial film forming method that usesa sputtering apparatus having a target electrode having a targetarranged thereon and a substrate holder provided with a heater electrodeand a bias electrode, and that epitaxially grows a +c polarity group IIInitride semiconductor thin film by sputtering on a sapphire substratearranged on the substrate holder, the epitaxial film forming methodcomprising: arranging the sapphire substrate on the substrate holder;and forming an epitaxial film of the +c polarity group III nitridesemiconductor thin film directly on the sapphire substrate arranged onthe substrate holder, by the sputtering by applying high-frequency powerto the target electrode and by applying high-frequency bias power to thebias electrode, wherein, during the forming an epitaxial film of the +cpolarity group III nitride semiconductor thin film directly on thesapphire substrate, the sapphire substrate at a predeterminedtemperature is maintained by the heater electrode, the high-frequencypower and the high-frequency bias power are applied so that frequencyinterference between the high-frequency power and the high-frequencybias power does not occur, and (A) the applying the high-frequency powerto the target electrode generates plasma on a front surface of thetarget such that the plasma causes the target to give off a group IIInitride molecule, while (B) the applying high-frequency bias power tothe bias electrode generates an electric field in a direction toward thesurface of the sapphire substrate in a region between the sapphiresubstrate and the plasma, such that the electric field orients apolarization, from a negative charge to a positive charge, of the groupIII nitride molecule toward the sapphire substrate supported by thesubstrate holder.
 2. The epitaxial film forming method according toclaim 1, wherein for frequencies of the high-frequency power and thehigh-frequency bias power, different frequencies are selected.
 3. Theepitaxial film forming method according to claim 1, wherein thehigh-frequency power and the high-frequency bias power, for which thesame frequency is selected, are applied so that a phase differencebecomes 180°.
 4. The epitaxial film forming method according to claim 1,wherein the bias electrode has a first electrode to which a positive DCvoltage is applied and a second electrode to which a negative DC voltageis applied, in a state in which the positive DC voltage is applied tothe first electrode and the negative DC voltage is applied to the secondelectrode, the sapphire substrate is electrostatically adsorbed by thesubstrate holder, and an epitaxial film of the +c polarity group IIInitride semiconductor thin film is formed on the sapphire substrate. 5.The epitaxial film forming method according to claim 1, wherein thehigh-frequency bias power is applied after the high-frequency power isapplied and before a film formation surface of the sapphire substrate iscovered by a crystal layer made of a group III nitride semiconductor. 6.A manufacturing method of a semiconductor light-emitting elementcomprising: forming a buffer layer of a semiconductor light-emittingelement by the epitaxial film forming method according to claim 1.